I’d have thought (though I have no actual experience of them) that the Goat Link only moves the derailleur mounting point, it doesn’t help much with respect to chain length.
The One Up RAD Cage nearly achieves what you’re after – it moves the top jockey wheel downwards when the cage is extended forward, so means the distance from the jockey wheel to the cassette is better throughout the range.
Really though, I’m surprised you’re having problems with this at all. A friend is managing a short cage zee on a 11-42t cassette, which suggests something isn’t right on yours…
Have you tried adjusting the B-screw to get a small amount of clearance in the biggest sprocket?
Can you send any photos from the side to show the mech angle and position? I’m thinking of the Productions Privee (Shan?) that had an incorrect shaped dropout leaving the mech at a funny angle.